Hitherto, a lithium ion battery (hereinafter, abbreviated to an LIB) has been known, and FIG. 1 is a conceptual diagram that illustrates an example of the LIB. The LIB is configured, for example, by an anode 1 having graphite as a negative electrode active material, a cathode 2 having LiCoO2 as a positive electrode active material, and an electrolytic solution 3. According to an electric field, lithium ions eluted from LiCoO2 of the cathode 2 into the electrolytic solution 3 move between electrodes in the electrolytic solution 3 and are maintained between graphite layers of the anode 1, whereby the function of a battery is implemented. However, the LIB has a configuration in which electric energy is stored according to chemical reactions in the electrodes. Accordingly, while the energy density is high, the power density is low. In addition, there is a problem in that deterioration according to charging/discharging is intense, and the product life is short.
Further, an electric double-layer capacitor (hereinafter, abbreviated as an EDLC) is known and FIG. 2 is a conceptual diagram that illustrates an example of the electric double-layer capacitor. The EDLC is configured, for example, by an anode 4 formed by activated carbon (abbreviated to AC), a cathode 5 formed by AC, and an electrolytic solution 6. Acted to an electric field, cations and anions disposed inside the electrolytic solution 6 are respectively moved to the surfaces of mutually-different electrodes, are attached to the surfaces, and form an electric double layer on the surfaces of the electrodes, whereby the function of a capacitor is implemented (Non Patent Literature 1). This EDLC has been reviewed for applications to a so-called supercapacitor (Non Patent Literature 2).
The supercapacitor is a capacitor of which the performance is improved more than a normal capacitor and is also called an ultracapacitor. Since ion molecules are configured to store electric charge, the supercapacitor can perform charging/discharging of 105 times or more, has a long product life, and has a low maintenance cost by employing a simple principle (Non Patent Literature 3). In addition, the supercapacitor has a power density higher than the LIB and thus, is expected to be applied to a memory backup system and an energy storing system such as an industrial power supply apparatus (Non Patent Literature 4). However, there is a problem in that the energy density of the supercapacitor is lower than that of the LIB by several tens of times.
A lithium ion capacitor (hereinafter, abbreviated to as an LIC) is also known. FIG. 3 is a conceptual diagram that illustrates an example of the lithium ion capacitor.
The LIC is configured, for example, by an anode 7 formed by Li-doped carbon, a cathode 8 formed by activated carbon, and an electrolytic solution 9. According to an electric field, lithium ions and negative ions disposed inside the electrolytic solution 9 are moved to the surfaces of mutually-different electrodes and are attached to the surfaces of the electrodes. Accordingly, the function of a capacitor is implemented. The Li-doped carbon of the anode 7, for example, is acquired by adding lithium ions between graphite layers.
FIGS. 4(a) and 4(b) are conceptual diagrams that illustrate an example of the operation principle of the LIC.
As illustrated in FIG. 4(b), in a state in which no electric field is applied to electrodes in the anode 7, for example, ionic-bonded Li+PF6− floats inside the electrolytic solution 9. Next, by applying an electric field to the electrodes, as illustrated in FIG. 4(a), lithium ions are inserted between graphite layers, PF6− ions are attached to the surface of activated carbon in the cathode 8, and an electric double layer 100 is formed on the surface of the activated carbon so as to be charged. When the application of the electric field to the electrodes is stopped, the ions are moved from the electrodes to the inside of the electrolytic solution and are ionic-bonded so as to be discharged. Such a cycle is repeated.
In this principle, as illustrated in FIGS. 4(a) and 4(b), the anode 7 formed by the Li-doped carbon serves as an energy source for insertion/drawing-out of the lithium ions. In this way, the energy density can be raised up to the level of a lithium ion battery. By adding lithium ions to the carbon in advance, the energy density can be further raised.
In addition, the cathode 8 formed by the activated carbon serves as a power source. In this way, the product life can be increased like the electric double layer capacitor, and the power density can be raised.
However, the energy density of the LIC is higher than that of the supercapacitor (EDLC) by several times but is still lower than that of the LIB by several tens of times.
Carboneous materials represented by activated carbon usually have properties of (1) a large surface area (SSA), (2) high electric conductivity, and (3) a uniform distribution of many pores having a pore diameter that can be accessed by electrolytic ions (Non Patent Literature 5).
Accordingly, various carboneous materials are reviewed for the use as electrodes by using such properties.
For example, a carbon nano tube (hereinafter, abbreviated to a CNT) that is one of carboneous materials has been drawing attention. However, the CNT has problems in that the accessibility of electrolytic ions is low, and the SSA is small (Non Patent Literature 6).
Meanwhile, graphene that is another carboneous material has a uniform distribution of many pores having a pore diameter that can be accessed by electrolytic ions, high electric conductivity, and a large SSA (26,302/g). It is reported that, by using such graphene, the energy storage performance can be improved (Non Patent Literatures 7 and 8) and reduced graphene formed by a chemical reduction method can reduce a manufacturing cost (Non Patent Literature 9). However, it is pointed out that a problem occurs in that the electric conductivity decreases due to the influence of restocking and a functional group according to a Van der waals force (Non Patent Literature 10).
The applications of graphene to capacitors have been reviewed from various viewpoints. For example, a supercapacitor including an electrode including an activated carbon layer and a graphene layer has been proposed (Patent Literature 1). Applications of graphene to a supercapacitor using graphene as a nano structure as the material of an electrode have also been reviewed (Patent Literature 2 and Non Patent Literature 11).
In addition, a CV curve of EDLCs has been reported to be symmetrical (Non Patent Literature 12).
Furthermore, the characteristics according to a Li insertion/non-insertion process have been reported (Non Patent Literature 13).
It is reported that, in a case where Urea-RGO is used as the material of the cathode, the specific capacitance is 126 F/g, and the energy density is 105 Wh/kg (Non Patent Literature 14).